1. Field of the Invention
The present invention relates to power converters and distribution schemes for power distribution. More specifically, the present invention relates to a transformerless output 3 phase power inverter topology and control method to facilitate AC power with very low DC offset content, high quality source, and low output total harmonic distortion (THD).
2. Background Art
Electric power distribution is a necessary component of systems that operate with electronic power or in the distribution of electronic power. For example, most electronic equipment is connected to a utility grid wherein power arrives in one form and is transferred and transformed into a form more suitable for the equipment.
The distribution of electric power from utility companies to households and businesses utilizes a network of utility lines connected to each residence and business. The network or grid is interconnected with various generating stations and substations that supply power to the various loads and that monitor the lines for problems. Distributed electric power generation, for example, converting power from photovoltaic devices, micro-turbines, or fuel cells at customer sites, can function in conjunction with the grid. Loads that are connected to the grid take the generated power and convert it to a usable form or for supplementing the grid.
An electric utility grid generally can also consist of many independent energy sources energizing the grid and providing power to the loads on the grid. This distributed power generation is becoming more common throughout the world as alternative energy sources are being used for the generation of electric power. In the United States, the deregulation of electric companies has spurred the development of independent energy sources co-existing with the electric utility. Rather than have completely independent energy sources for a particular load, these alternative energy sources can tie into the grid and are used to supplement the capacity of the electric utility.
The number and types of independent energy sources is growing rapidly, and can include photovoltaic devices, wind, hydro, fuel cells, storage systems such as battery, super-conducting, flywheel and capacitor types, and mechanical means including conventional and variable speed diesel engines, Stirling engines, gas turbines, and micro-turbines. In many cases these energy sources can sell the utility company excess power from their source that is utilized on their grid.
Each of these independent energy sources needs some type of power converter that feeds energy to the grid or used to directly power the various loads. There must also be some means to provide protection when the grid becomes unstable. In most scenarios the utility company is still the main power source and in many cases controls the independent source to some extent.
A problem with the prior art system is that the distribution system is subject to non-linear, high harmonic content and unbalanced loading. This is especially true where the distributed generation system operates independent of the utility grid, and must therefore provide all of the load required harmonic currents. In distributed power applications, high harmonic content or unbalanced loads may lead to utility grid instability, resonances or other unanticipated distribution system behavior that may cause catastrophic failure of the distribution system components. Such a failure can result in damage to equipment and possibly personal injury.
Power converters such as inverters are necessary in modem power systems for the new energy generating devices such as photovoltaic devices, micro-turbines, fuel cells, superconducting storage, etc., that generate AC or DC electricity that needs to be converted to a conditioned AC for feeding into the power grid or for direct connection to loads.
Grid independent DC-AC inverters generally behave as sinusoidal voltage sources that provide power directly to the loads. This type of power distribution architecture is generally required to provide power to both 3 phase and single phase, or line to neutral connected loads. Typically, 3 phase power inverters meet this 3 phase+neutral requirement by isolating the power inverter from the loads with a delta-wye power transformer.
Grid connected DC-AC inverters generally behave as a current source that injects a controlled AC sinewave current into the utility line. The controlled AC current is generated in sync with the observed utility zero crossings, and may be exactly in phase, generating at unity power factor where upon real power only is exported. It is also possible to generate a variable amount out of phasexe2x80x94at other than unity power factor where upon real and reactive power is exported to the grid. An effective change in reactive power output can be made by either phase shifting the output current waveform with respect to voltage or by creating an assymetric distortion to the output current waveform.
Whether grid connected or grid independent, typical delta-wye transformer isolated 3 phase power inverters demonstrate poor output waveform THD when connected to non-linear loads. This is particularly true in the case of even order harmonic currents (2nd, 4th, 6th, 8th etc.). Specifically, typical power transformers common to most power distribution systems demonstrate a tendency to saturate when exposed to even order or DC content load generated non-linear currents. This causes the transformer""s impedance to instantaneously decrease, thereby allowing excessive asymmetric currents to flow through the transformer""s windings, while decreasing the power actually coupled from primary to secondary. A variety of factors define how steep this saturation transition will occur, including magnetic core material and construction, magnitude of even order harmonics, and transformer operating temperature. At the least, very poor output power quality, nuisance circuit breaker tripping, increased distribution system components losses and increased operating temperatures will be observed.
Although distribution system transformer saturation is not as likely to occur in utility grid connected systems (due primarily to the utility grid""s typically lower impedance than the grid connected inverter system), distortion and instability may still occur. This problem is greatly aggravated where power inverters act as xe2x80x9cstand alonexe2x80x9d voltage sources, where the inverter comprises the only power source to the local distribution system.
These problems are currently solved in the distribution system by over sizing the distribution transformers. For power inverters, expensive gapped core type isolation transformers are commonly employed to decrease the power conditioning system susceptibility to even order harmonic currents, as well as isolate inverter generated DC voltage offsets from the distribution system. The increased cost and space requirements for the isolation transformers are problems that are well known in the industry.
Inverters that perform DC-AC conversion function, and are connected to the grid, are known as xe2x80x9cUtility-Interactive Invertersxe2x80x9d and are the subject of several US and international codes and standards, e.g., the National Electrical Code, Article 690xe2x80x94Photovoltaic Systems, UL 1741, Standard for Photovoltaic Inverters, IEEE 929xe2x80x94Recommended Practice for Utility Interface of Photovoltaic (PV) Systems.
Pulse width modulator (PWM) inverters are used in three phase bridges, H-bridges, and half-bridge configurations. The bus capacitors, typically electrolytic, consist of two or more capacitors connected in series that are fed from a rectifier or actively switched front end section.
In order to reduce the aforementioned problems, attempts have been made to produce an improved dispensing system. The prior art systems have general short-comings and do not adequately address the aforementioned problems.
What is needed is a means of efficiently operating power inverters, especially for non-linear, high harmonic content, and/or unbalanced loads. This design must also be cost effective to manufacture and implement, and allow for easy incorporation into current designs.
The present invention has been made in consideration of the aforementioned background. It is therefore an object of the present invention to provide a 3 phase power inverter topology and control method to facilitate very low DC offset voltage, high quality, low output THD operation of three phase power inverters. Particularly when applied to non-linear, high harmonic content, and/or unbalanced loads, common in modem power distribution systems, both commercial and industrial. The topology of the present invention eliminates the requirement of an external isolation transformer, and, yields a considerable cost savings and reduction in space requirements and overall weight. An isolation transformer is normally required for typical three phase inverters when applied to xe2x80x9cdistributed powerxe2x80x9d or xe2x80x9cpower generationxe2x80x9d applications that require the power source to provide unbalanced, harmonically rich phase currents with very low output voltage THD. Although isolated gate bipolar transistor (IGBT) power switches are the preferred embodiment in 480V inverter applications, other power switches used in the lower voltage applications (FET""s for example) are within the scope of the invention.
Another object of the invention is to provide 3 phase output power at a very low output impedance. This allows for superior output waveform quality (lower voltage THD) than is possible with a typical transformer isolated power inverter. Excessive voltage distortion in inverter based power systems is caused by several factors, including single phase or unbalanced loading of three phase 4 wire power inverter systems, especially when the neutral is connected to the center point between two series capacitors. Another typical cause is non-linear, high harmonic current content loads on three phase systems, particularly those that contain a high magnitude of even order, triplen or DC harmonic current components.
The present invention applies not only to DC-AC inverters, but also to many other methods of electric power conversion, such as static inverters, and rotary converters (DC-AC motor-generator sets that convert DC electricity to AC electricity), cycloconverters and AC to AC motor generator sets (convert AC electricity to AC electricity), and mechanical generators that convert mechanical energy to AC electrical energy.
The control printed circuit board (PCB) of the present invention acts as a digital signal processor (DSP) based digital PWM AC voltage and/or current regulator, with independent phase voltage and current control loops. Independent voltage loops control the line to neutral voltage of each phase output. In one embodiment voltage feedback is provided by three individual resistance isolated differential operational amplifier circuits that incorporate very large integrator capacitors to boost DC gain and thereby enhance DC offset voltage rejection. Independent current loops control the output phase currents. Current feedback is provided by isolated hall sensors and precision operational amplifier circuits.
Both the voltage and the current loops have digitally selected proportional and integral terms, and the feedback circuits have analog phase lead and filter circuits for optimum system tuning. Thus, precise closed loop transient performance is accomplished
When connected to the utility grid, use of large capacitor integrators in inverter voltage feedback circuits will tend to cause a gradual phase shift of the voltage feedback circuit based PLL""s (phase locked loops) that are commonly used by inverter controls to establish correct synchronization to the utility grid phase angle. This causes output power factor to vary, and will eventually lead to a loss of synchronization. To solve this problem, the DSP closes bi-directional switches across these large integrator capacitors when the inverter is utility grid connected. Further, these switches are used to discharge the integrator capacitors after a system fault and prior to stand-alone voltage mode startup. This prevents a variety of xe2x80x9cintegrator wind upxe2x80x9d problems from occurring.
As described herein, grid independent DC-AC inverters behave as sinusoidal voltage sources and provide power directly to the loads. These prior art power distribution schemes generally require providing power to both 3 phase and single phase or line to neutral connected loads. The 3 phase power inverters for DC-AC accomplish this 3 phase+neutral requirement by isolating the power inverter from the loads with a delta-wye power transformer. For 3 phase inverters equipped with a balanced dual boost regulator and the transformerless output 3 phase power inverter topology and control described herein, this costly transformer is unnecessary. The dual boost regulator is further described in the U.S. Patent Application entitled Split-Fed and Balanced Dual Boost Regulator filed Dec. 7, 2000. The transformerless 3 phase+neutral inverter power output can therefore achieve lower overall output impedance than an isolation transformer equipped inverter. Thus, lower output voltage distortion is achieved, particularly when the inverter is connected to non-linear loads.
Three phase power inverter systems are commonly exposed to a mixture of linear, non-linear (harmonic), and transformer loads. Transformers are particularly sensitive to DC content of the AC fundamental output voltage. The balanced dual boost regulator, as well as the high frequency center tapped transformer (diode rectified) topologies, forces the output inverter DC bus capacitor voltages to remain balanced thereby dramatically reducing the DC offset of the neutral point (between the upper and lower output capacitors). When combined with the 3 phase inverter transformerless topology and control method, a three phase power inverter maintains high quality output waveforms suitable for transformer loads, even when exposed excessive non-linear loads. Another benefit of the balanced dual boost regulator is that the three phase inverter control algorithms and feedback circuits can be greatly simplified due to the very good stability of the DC bus voltage.
The 3 phase power inverter topology and control method described herein eliminates the need for expensive and oversized isolation transformers both in the inverter power system, as well as throughout the local power distribution system. This reduces power distribution and inverter power conditioning or generating system costs, while simultaneously enhancing overall tolerance of even order and or DC currents.
Since AC distribution system backup requires the use of 3 phase inverters to temporarily feed power into the distribution system, the 3 phase power inverter topology and control method will automatically provide a low output impedance, very low DC offset content ( less than 20 m VDC), AC output waveforms. Such systems are suitable for use with flywheel power inverters.
A further object of the invention is to provide 3 phase 4 wire output power that is more efficient and at substantially lower cost than other transformerless power inverters. This is especially true when compared to a switched neutral type approach that uses a fourth xc2xd bridge or neutral phase connected to the DC link of a typical 3 phase power inverter and is PWM controlled to hold the neutral phase output a zero volts DC potential. Thus a switched neutral allows for neutral return currents to the appropriate side of the DC link.
The main disadvantage with the switched neutral strategy is that the entire DC link voltage is applied across the neutral phase output LC, or PWM ripple filter. This is distinct from the 3 phase inverter transformerless topology and control method described herein, in that no output LC PWM ripple filter is required in the dual boost equipped 3 phase transformerless topology. Further, the voltage switching ripple applied across an output filter inductor creates core losses proportional to the voltage applied across said filter. That is, the difference between the switched DC voltage and the output voltage, it can be seen that the neutral PWM filter will incur greater core losses than even the individual output phase PWM filters. In addition, in a typical 3 phase, 4 wire output power inverter with balanced phase loads, the neutral current (INeutral) is greater than any one of the phase currents (Iphase out). This further decreases the inverter efficiency by increasing the IGBT and PWM filter losses. These problems are averted in the dual boost equipped, 3 phase power inverter topology and control method described herein.
INeutral=3* Iphase out
And yet an even further object of the invention is a very efficient, low cost, single or polyphase transformerless power inverter and distribution system for use in automotive, or other very cost sensitive applications. With the expected revolution in automotive design induced by the introduction of fuel cell based technologies, use of the balanced dual boost topology may be indicated. The possible balanced dual boost applications become numerous when applied to various xe2x80x9chybridxe2x80x9d battery/fuel cell/flywheel based main power sources as well as various auxiliaries (pumps, heaters, small motor drives, etc.)
One object of the invention is a transformerless power inverter system for generating a balanced, regulated AC output, comprising a DC-DC converter for regulating a DC source, an inverter section for generating the regulated AC output from the DC source, a control section connected to the inverter section for pulse width modulating the inverter, wherein the control section has a voltage loop and a current loop for each phase of the AC output, and wherein the control section has a means of processing.
Another object is a transformerless power inverter system, further comprising a rectifier section connected to a three-phase 4 wire source for generating the DC source.
Additionally, an object includes a transformerless power inverter system wherein the regulated AC source is single phase. Also, a transformerless power inverter system, wherein the regulated AC source is three-phase.
Additionally, an object is a transformerless power inverter system, further comprising a digital signal processor in the control section.
A further object is a transformerless power inverter system, further comprising a three-phase filter connected to the regulated AC source.
And yet another object is a transformerless power inverter system, wherein the DC-DC converter is a dual boost regulator.
Another object is a transformerless power inverter system, further comprising an integrator bypass switch in the control section for discharging a plurality of integrators.
An object of the invention is a transformerless power inverter system for generating a balanced, regulated AC output, comprising an AC power source, a rectifier section for converting the AC source to a DC source, a DC-DC converter for regulating the DC source, an inverter section for generating the regulated AC output from the DC source, a control section connected to the inverter section for pulse width modulating the inverter, wherein the control section is a voltage loop and a current loop with a means of processing.
Yet another object is a transformerless power inverter system, wherein the DC-DC converter is a center tapped HF transformer.
An object of the invention includes a method for generating a regulated AC output from an inverter, comprising the steps of calculating a voltage feedback for each phase of the regulated output, calculating a current feedback for each phase of the regulated output, comparing a voltage command to the voltage feedback to produce a voltage error, multiplying the voltage error by a voltage proportional and integral term to produce a proportioned voltage signal, comparing the proportioned voltage signal to the current feedback to calculate a current error, multiplying a current error by a current proportional and integral term to produce a proportioned current signal, and calculating a pulse width modulated signal for the inverter.
A final object of the invention is a method for generating a regulated AC output, further comprising a step of closing a bypass switch to discharge a plurality of integrators.
Other objects, features and advantages are apparent from description in conjunction with the accompanying drawings.